We establish the solvability of second order divergence-type parabolic systems in Sobolev spaces. The leading coefficients are assumed to be merely measurable in one spatial direction on each small parabolic cylinder with the spatial direction allowed to depend on the cylinder. In the other orthogonal directions and the time variable, the coefficients have locally small mean oscillations. We also obtain the corresponding W 1 p -solvability of second order elliptic systems in divergence form. This type of system arises from the problems of linearly elastic laminates and composite materials. Our results are new even for scalar equations, and the proofs differ from and simplify the methods used previously in [H. Dong and D. Kim, Arch. Ration. Mech. Anal., 196 (2010), pp. 25-70]. As an application, we improve a result by Chipot, Kinderlehrer, and Vergara-Caffarelli [Arch. Ration. Mech. Anal., 96 (1986), pp. 81-96] on gradient estimates for elasticity system Dα(A αβ (x 1 )D β u ) = f , which typically arises in homogenization of layered materials. We relax the condition on f from H k , k ≥ d/2, to Lp with p > d.
Introduction.In this paper we prove the unique solvability of divergencetype fully coupled parabolic and elliptic systems in Sobolev spaces when the leading coefficients are in the class of variably partially bounded mean oscillation (BMO) functions. The distinguishing feature of variably partially BMO coefficients is that they are allowed to be very irregular with respect to one spatial direction, and the spatial direction needs to be determined only locally. Thus the systems studied in this paper may model deformations in composite media as fiber-reinforced materials (see, e.g., [8] and [28]).There are many papers concerning elliptic and parabolic equations/systems in Sobolev spaces with vanishing mean oscillation (VMO) or BMO-type coefficients. Chiarenza, Frasca, and Longo first proved the interior estimate for nondivergence form elliptic equations with VMO coefficients [6]. Then the solvability of elliptic and parabolic equations in Sobolev space was presented in [7] and [5]. These are the earliest papers about nondivergence-type equations with VMO coefficients. For divergence-type equations with VMO/BMO coefficients, similar results were obtained in [11,2], and later in [3,4]. On the other hand, in [25,26] Krylov gave a unified approach to investigating the L p -solvability of both divergence and nondivergence form parabolic and elliptic equations with coefficients that are BMO in the spatial